1. Field of the Invention
The present invention relates to a method and an apparatus for forming a crystalline silicon thin film.
2. Description of Related Art
Polycrystalline silicon, nanocrystalline silicon and others have been known as crystalline silicon, and these can be used for various purposes.
Polycrystalline silicon thin films have been used, e.g., as materials of TFT (thin film transistor) switches arranged at pixels in liquid crystal display devices as well as materials of various integrated circuits, solar cells and others. It has been expected to use nanocrystalline silicon in nonvolatile memories, light emitting elements and optical sensitizers.
For example, known methods of forming the polycrystalline silicon thin film include (1) a method wherein a deposition target substrate is kept at a temperature of 800 deg. C. or higher, and a CVD method such as a plasma CVD method or a PVD method such as a sputtering vapor deposition method is effected at a lower pressure (see, e.g., JP5-234919A and JP11-54432A), and (2) a method wherein an amorphous silicon thin film is formed at a relatively low temperature by a method among various CVD and PVD methods, and thereafter a heat treatment at about 1000 deg. C. or a long-time heat treatment at about 600 deg. C. is effected on the amorphous silicon thin film as a post-treatment (see, e.g., JP5-218368A).
Such a method is also known that laser annealing is effected on an amorphous silicon film to crystallize the film (see, e.g., JP8-124852A).
In addition to the above, such a method has been proposed that plasma of a gas prepared by diluting a silane-containing gas such as monosilane (SiH4) or disilane (Si2H6) with hydrogen or silicon fluoride (SiF) is prepared and, in this plasma, a crystalline silicon thin film is directly formed on a substrate at a low temperature of about 500 deg. C. or lower (see, e.g. JP 2000-195810A).
By the way, crystalline silicon thin films are preferably those having a surface subjected to terminating treatment with oxygen, nitrogen or the like. The term “terminating treatment” used herein refers to a treatment wherein, e.g. oxygen and/or nitrogen is bonded to the surface of the crystalline silicon thin film to give a (Si—O) bond, a (Si—N) bond, a (Si—O—N) bond or the like.
The oxygen bond or nitrogen bond formed by such terminating treatment can function so as to compensate a defect, e.g., dangling bond, on the surface of the terminally untreated crystalline silicon thin film and thereby substantially give a suppressed state of the defect as a whole. When employed as materials of electronic devices, the terminally treated crystalline silicon thin films can achieve improvements in the properties required for the electronic devices.
For example, when used as a TFT material, the terminally treated crystalline silicon thin film enhances an electron mobility in the TFT, reduces OFF-current and increases reliability as to scarce variation in volt-ampere characteristic even in a long-time use of the TFT.
The terminating treatment is referred to in the method for forming a nanocrystalline silicon structure terminally treated with oxygen or nitrogen as described in JP 2004-83299A.
However, among conventional methods for forming a crystalline silicon thin film, a method involving heating a deposition target substrate to a high temperature requires a highly heat-resistant expensive substrate (e.g., silica glass substrate) as a substrate on which a film is formed. For example, it is difficult to form a crystalline silicon thin film on, e.g. an inexpensive low-melting point glass substrate having a heat resistant temperature of 500 deg. C. or lower (typically non-alkali glass substrate). This increases the production cost of crystalline silicon thin films in terms of the substrate cost. In the case of heat-treating an amorphous silicon film at a high temperature, the same problem is posed.
When an amorphous silicon film is subjected to a laser annealing process, a crystalline silicon film can be formed at a relatively low temperature. In this case, however, a laser irradiation step is required, and laser beams of an extremely high energy density must be emitted. For these and other reasons, the producing cost of the crystalline silicon thin film in this case is likewise high.
Further it is difficult to uniformly irradiate each part of the film with laser beams, and the laser irradiation may cause desorption of hydrogen, possibly resulting in a rough surface of the film so that difficulty is incurred in obtaining a crystalline silicon thin film of good quality.
In the method wherein a crystalline silicon thin film is formed directly on the substrate at a relatively low temperature with the plasma formed of a silane-containing gas diluted with a hydrogen gas or silicon fluoride (SiF), a film deposition rate is lowered because of using a silane-containing gas diluted with hydrogen or the like. The monosilane gas would incur a risk of causing a spontaneous ignition in the air.
The method for forming a nanocrystal silicon structure described in JP 2004-83299A presents the same problem as in formation of the conventional crystalline silicon thin film described above in that a silicon thin film of nanometer-scale thickness comprising silicon minute crystals and amorphous silicon is formed prior to the terminating treatment by a thermal catalysis reaction of a gas containing a hydrogenated silicon gas and a hydrogen gas or by applying a high-frequency electric field to a gas containing a hydrogenated silicon gas and a hydrogen gas to form plasma to be used for film formation.